The present invention relates to pressure sensitive adhesives (PSAs) and, in particular, to silicone pressure sensitive adhesives (SPSAs) having a reduced volatile fraction content, to the resulting adhesives and articles bearing same, and to process for reducing the low molecular weight fraction content of a cured PSA. In a preferred embodiment the present invention relates to an article comprising a low volatility SPSA disposed on a substrate.
In general, PSAs are comprised of a resin component, a polymer component and a curing agent component, in addition to various optional components, such as a solvent. One or more of these components contain low molecular weight fractions, the presence of which in the cured PSA can have a deleterious effect on the usefulness of the PSA.
For example, when a PSA is exposed to the hard vacuum of space these low molecular weight fractions can evaporate, during the lifetime of a vehicle to which the PSA has been applied, and can condense on cold surfaces thereof. The performance of optical surfaces on the vehicle, in particular, are significantly degraded by such deposits.
For another example, when a PSA has been applied to a surface these low molecular weight fractions can migrate at standard temperatures and pressures from the position where the PSA has been applied to adjoining regions where they exert undesirable effects, such as surface contamination.
Clearly, there exists a need for PSAs having a reduced content of low molecular weight fractions.
An uncured PSA is typically applied to a substrate, such as a backing or a release liner, and cured to a crosslinked polymer network before it is used in its ultimate application. Consequently, the low molecular weight fractions thereof are impossible to remove by evaporation, even at reduced pressure and elevated temperature. On the other hand, removal of the low molecular weight fractions from the components of the PSA prior to preparing the uncured PSA or from the uncured PSA is of no avail because the chemical reaction that is used to cure the PSA and/or the solvent that is used to aid in the application of the PSA are sources of low molecular weight fractions.
McHugh and Krukonis, "Supercritical Fluid Extraction", Chapter 9, Butterworth Publishers, 1986, have reported that supercritical fluid extraction (SCFE) has been use to extract low molecular weight materials from silicone oil to an extent unattainable by molecular distillation. These authors also report that, while the SCFE process is an adjustable process, leading to selective dissolving powers, its efficacy is also highly dependent on the molecular structure of silicone oil. The authors are silent as to the application of SCFE to cured silicone compositions, such as SPSAs.
Copley et al., European Patent Application 255,226, discloses the use of SCFE to fractionate a silicone resin into fractions of varying molecular weight and the use of those fractions to prepare SPSAs. Although one or more of these fractions may be sufficiently free of low molecular weight fractions to meet many low-volatility requirements, the other components of the SPSAs of Copley et al. are not.